Solid state laser



March 10, 1970 J. E; GEUSIC YETAL 3,500,232

SOLID STATE LASER Filed April 1, 1966 J. E. GEUS/C INVENTOR L.F. JOHNSON By LG. VAN U/TERT A T TORNE V United States Patent M 3,500,232 SOLID STATE LASER Joseph E. Geusic, Berkeley Heights, Leo F. Johnson, Bedminster, and Le Grand G. Van Uitert, Morris Township, Morris County, N.J., assignors to Bell Telephone Laboratories Incorporated, New York, N.Y., a corporation of New York Filed Apr. 1, 1966, Ser. No. 539,461 Int. Cl. H01s 3/16; C09k J/04, N68

US. Cl. 33194.5 9 Claims ABSTRACT OF THE DISCLOSURE The emission intensities of fluorescent compositions containing thulium are enhanced by the incorporation therewith of ytterbium ions alone or in combination with chromium ions. Additionally, solid state lasers utilizing rare earth aluminum or gallium garnet, doped with ytterbium and thulium are found to be capable of CW room temperature operation.

This invention relates to luminescent materials and, more particularly, to luminescent compositions containing thulium ions in combination with ytterbium ions, alone or in combination with chromium ions, such compositions being of particular interest for use in laser applications.

Recently, considerable interest has been generated in a class of devices termed laser which are capable of amplifying or generating coherent electromagnetic wave energy in the optical frequency range. Devices of this type which are described, for example in U.S. Patent 2,929,922 granted to Schawlow and Townes, are considered to be operable over the spectral range from far infrared to ultraviolet, a frequency range of about 10 cycles.

In principle, these devices are directly analogous to the microwave maser. In a mechanism involving the three level form required for continuous wave (CW) operation, pump frequency of at least the Planck frequency corresponding with a separation between a ground and excited state together with a subsequent relaxation to a metastable state sufficient to result in at least an equal population distribution between this metastable and some lower state fulfills the negative temperature requirements. Application of wave energy of the frequency corresponding with the energy separation between such metastable and lower state in accordance with Planck's law, as in the microwave analog, results in the stimulation of energy of the same frequency in phase with the stimulating signal. Pro viding the relaxation rate from the metastable to the lower state is suitably slow and providing means for a preferred mode operation, the resulting energy output is single mode and coherent.

In accordance with the present invention, it has been discovered that the emission intensities of fluorescent compositions containing thulium are significantly enhanced by the further incorporation therein of ytterbium ions, alone or in combination with chromium ions the resultant composition evidencing an unusual combination of characteristics indicating their advantageous use in an optically pumped laser.

In a preferred embodiment of the present invention, it has been found that high fluorescent efiiciency results in compositions of the general formula a b e) 12 wherein M may be yttrium, gadolinium, lutecium or mixtures thereof, Me may be trivalent gallium or aluminum alone or in combination with chromium in an amount ranging from 0.161.6 atom percent of the gallium or aluminum and the sum of a+b+c equals 3, a ranging from 0-2.949, b ranging from 0.05-2.999 and c ranging from 0.0008l.0.

The invention will be more fully understood by reference to the following detailed description taken in conjunction with the accompanying drawing wherein:

The figure is a cross-sectional view of a pump cavity and laser device utilizing the composition of the present invention.

With reference now to the figure, there is shown an apparatus including an elliptical cavity 11 containing Yb-Tm-YAG laser 12 and pump 13, each at a focal point of the ellipse formed by cavity 11. Ends 14 and 15 of laser 2 are ground and polished so as to be optically flat and parallel and are silvered so as to provide reflective layers, the former providing for complete and the latter for partial reflectance so as to permit the escape of coherent radiation. Coherent emission, shown as arrow 16, emanates from partially reflecting end 15. During operation, laser 12 is maintained at liquid nitrogen temperature.

It has been indicated that the materials utilized herein are capable of emitting energy of narrow line width in the light spectrum at a defined frequency. More specifically, the described compositions utilize thulium as the active ion, thulium being known to be characterized as having emission lines at about two microns. Trivalent thulium, however, evidences an absorption spectrum which consists of several relatively weak bands. Thus, it is diflicult to pump thulium easily with black body radiation as such sources possess a broad spectrum and the material can only absorb over a narrow region. This difficulty is overcome by the use of trivalent ytterbium which manifests relatively strong absorption bands in the one micron region near the peak in emission of a 3200 K. black body emitter and transmits energy so absorbed to the thulium ion, thereby increasing the efficiency of utilization of the pump radiation. For the purposes of the present invention, a tungsten lamp operating at approximately 3000 K. is a preferred pump source. Since the energy transfer mechanism between the ytterbium ions and the thulium ions is essentially environment independent for those environments transparent in the 0.9-3 micron region, any environment in which thulium and ytterbium ions fluoresce will exhibit the energy transfer required for enhancing laser action.

Exemplary environments for the purpose of the present invention may be represented by the following general formulae: ABO, wherein A may be calcium, strontium, barium or lead and B is molybdenum or tungsten; MC O wherein M may be yttrium( gadolinium, lutec'jium or mixtures thereof and C may be titanium, tin silicon, germanium or zirconium; MDO wherein M is defined as above and D may be aluminum, gallium, scandiurn, lutecium, lanthanum, yttrium, boron or gadolinium; the garnet system alluded to hereinabove, et cetera.

It has been determined that enhanced thulium fluorescence in any of the noted host environments results when the compositions'include at least one trivalent thulium ion per 10,000 cations and at least 60 trivalent ytterbium ions per 10,000 cations. On an atom per cent basis, this corresponds with fluorescent compositions in which at least 0.01 percent of the cations present are thulium ions and 0.6 percent of the cations present are ytterbium ions. Additionally, chromium may be present in an amount ranging from 0.1-1 per cent of the cations present in the compositions.

As noted above, preferred compositions of the present invention are of the general formula (M Yb Tm )Me O wherein M is selected from among yttrium, lutecium, gadolinium or mixtures thereof, Me is selected from among gallium and aluminum, alone or in combination with chromium in an amount ranging from 0.16-1.6 atom per cent of the aluminum or gallium, a ranges from 2.949, b ranges from 0.05-2.999 and c ranges from 0.0008-1.0, the sum of a+b+c equalling 3. While laser action may result at levels lower than the indicated minima for ytterbium and thulium, the noted values are considered expeditious for practical operation. The use of quantities of ytterbium or thulium exceeding the indicated maxima is less advantagous. For the purposes of the present invention preferred ranges are as follows: a ranging from 1.3-2.87, b ranging from 0.1-1.5 and c ranging from 0.0l0.6. A general preference is found to exist for compositions in, this system wherein Me is aluminum. With regard to the M content, it is noted that the absence thereof does not preclude laser action. The maximum content is determined by practical considerations The described garnet system has been found to be particularly advantageous in that it permits CW room temperature operation at low thresholds. Although CW operation can be achieved at room temperature, it is not to be considered limiting and it may reasonably be predicted that CW operation may be achieved at higher temperatures. The advantageous characteristics of the described system may also be used for pulsed operation. Experiments conducted by the inventors herein have shown a pulse threshold of 360 joule-inches at room temperature for the Y2 47Yb0 5Tm0 o3A1 O composition. From htis value a CW threshold of less than 400 watts for a one inch rod may be predicted.

Compositions found to be possessed of the requisite characteristics for operation in accordance with the invention have been grown by conventional pulling techniques and by spontaneous nucleation in accordance with the procedure described and claimed in copending appli cation Ser. No. 508,151, filed Nov. 16, 1965.

Lasers of the general type illustrated in the figure have been operated utilizing as the active material compositions of. the invention. An illustrative example of such laser operation is given below.

EXAMPLE An yttrium aluminum garnet containing ytterbium and thulium was grown by the flux technique described in the copending application alluded to hereinabove by melting together 3536 grams PhD, 4322 grams PbF 197 grams B 0 417 grams Y O 147 grams Yb O 8.7 grams Tm O and 690 grams A1 0 in a one gallon size platinum covered container. The temperature of the system was raised to 1300 C. and cooling initiated at the rate of /z/hour until a temperature of 950 C. was reached. Following, the flux was drained and the crystals cooled to room temperature. Finally, rods were cut from the resultant crystals which were of the composition The resultant crystal was employed in a device of the type shown in the figures, producing intense coherent emission having a wavelength of approximately 1.88 and 2.01 microns at liquid nitrogen temperature and at 2.01 microns at room temperature. The threshold pulse required was 50 joule inches at 77 K. and 360 joule inches at 300 K., the estimated CW threshold being 50 and 360 watts, respectively. Greatly reduced thresholds are to be expected upon increasing the Tm content.

While the invention has been described in detail in the foregoing specification and the drawing similarly illustrates the same, the aforesaid is by way of illustration only and is not restrictive in character. The several modi- -fications which will readily suggest themselves to persons skilled in the art are all considered within the scope of this invention, reference being had to the appended claims.

What is claimed is:

1. A luminescent composition of matter of the general formula (M Yb TM )Me O wherein M is selected from the group consisting of yttrium, luteciurn, gadolinium and mixtures thereof, Me is selected from the group consisting of gallium and aluminum, a ranges from 0.02.949, b ranges from 0.054.999 and c ranges from 0.0008-1.0, the sum of a+b+c equalling three.

2. A composition in accordance with claim 1 wherein a ranges from 1.3-2.87, b ranges from 0.11.5 and c ranges from 0.010.6. 4

3. A composition in accordance withclaim '1 wherein Me is aluminum.

4. A composition in accordance with claim 1 wherein Me includes chromium in an amount ranging from 0.16-1.6 atom percent thereof.

5. A laser comprising an active element consisting essentially of a composition of matter of the general formula a b c) M65012 wherein M is selected from the group consisting of yttrium, gadolinium, lutecium and mixtures thereof, Me is selected from the group consisting of aluminum and gallium, a ranges from 0.02.949, b ranges from 0.05-2.999 and c ranges from 0.0008-1.0, the sum of a+b+c equalling three.

6. A device in accordance with claim 5 wherein Me is aluminum.

7. A device in, accordance with claim 5 wherein a ranges from 1.3-2.-87, b ranges from 0.1-1.5 and c ranges from 0.01-0.6.

8. A device in accordance with claim 5 in which the active element is rod shaped and in which both ends are coated in such manner that one is totally reflecting and the other is partially reflecting for electromagnetic energy of the order of two microns in wavelength.

9. A laser oscillator capable of CW operation comprising an active medium consisting essentially of a composition of matter of the general formula a b c) M65012 wherein M is selected from the group consisting of yttrium, lutecium, gadolinium and mixtures thereof, Me is selected from the group consisting of aluminum and gallium, a ranges from 0.02.949, b ranges from 0.05-2.999 and c ranges from 0.0008-1.0, the sum of a-I-b-I-c equalling three, and pumping means for pumping said composition so that a population inversion is produced between a pair of optically connected energy levels.

References Cited UNITED STATES PATENTS 3,252,103 5/1966 'Geusic et al. 2-52301.4 X 3,405,371 10/1968 Johnson et a1 252--301-.4

OTHER REFERENCES Hu-tchings et al., Physical Review Letters, vol. 11(5), pp. 187-90 (1963 HELEN M. MCCARTHY, Primary Examiner J. COOPER, Assistant Examiner U.S. Ol. X.R. '2'52--301.4 

